Silverstripe stores file manifest information and parsed template files in your temp folder (/tmp/ maybe) - just remove everything in this folder. You can also set the tmp folder that SilverStripe uses by defining the constant TEMP_FOLDER in your _config.php file:

When I reread your last post, Hamish, am I correct in saying that I can create a folder on my site called say 'afstemp' and define it as the temp folder that ss shoiuld use?
If so I could easily find this and clear it manually - I guess??

I'm back in business.
It turns out that the webservers I'm on require some time before they effect the changes that I made to the sqlDB and folders etc.
When I retried the Dev/Build and ?flush=1 all now seems to do what it apparently should.

I've also now added a couple of modules and they appear to be good too so I'm fingers crossed for now and it's on with page creation etc.